1) Field of the Invention
The present invention relates to a process and an apparatus for vacuum degassing molten steel in a vacuum treatment vessel such as an RH vacuum treatment vessel, a DH vacuum treatment vessel, a ladle vacuum treatment vessel comprising a casing for encasing a ladle and a top cover for shielding the ladle from the surrounding atmosphere and a treatment vessel immersed in a ladle, and relates to an apparatus for vacuum degassing molten steel, which is used in a secondary refining process.
2) Prior Art
Recently, mass-produced high grade steel has been often subjected to a secondary refining treatment in a vacuum treatment vessel, and above all such operations as to supply an oxygen gas to molten steel in an RH vacuum treatment vessel to positively decarburize the moten steel or as to positively heat the molten steel have been widely carried out. However, such vacuum treatments have such problems as a decrease in the temperature of molten steel and deposition of much molten steel on the inside walls of the RH vacuum treatment vessel.
Heretofore, it has been proposed to provide a heater of electrical resistance type in the RH vacuum treatment vessel, but the conventional heater of electric resistance type is not enough to prevent the decrease in the temperature of molten steel or the deposition of molten steel. Furthermore, the conventional heater of electrical resistance type suffers from a high capital investment, a high electrode consumption per unit production and a high power cost, resulting in higher decarburization treatment cost.
According to the present inventor's knowledge, the decrease in the temperature of molten steel and deposition of molten steel can be prevented to some extent by thoroughly preheating the inside of an RH vacuum treatment vessel in which molten steel has not been treated yet and which is on standby. However, it has problems such that the heating capacity of the conventional heater of electrical resistance type is not enough and electrode and power costs are so high as to increase the RH vacuum treating cost.
Japanese Patent Application Kokai (Laid-open) No. 53-81416 discloses a process comprising adding Al, Si and the like into molten steel and heating the molten steel by injecting an oxygen gas into the molten steel in a vacuum treatment vessel. However, it has such problems that expensive materials such as Al, Si and the like must be used and there is a high chance for deposition of molten steel on the inside wall of the vacuum treatment vessel.
U.S. Pat. No. 4,979,983 discloses a process for injecting an oxygen gas onto the molten steel surface in a vacuum treatment vessel and combusting the CO gas generated from the molten steel in the vacuum treatment vessel through reaction with the injected oxygen gas. However, it has such problems that the heat source is only the CO gas generated from the molten steel, and thus the steel species to be treated is limited only to the steel species to be decarburized, and the heating capacity also depends on the amount of generated CO gas. Thus, there is an insufficient case for preventing the decrease in the temperature of molten steel, and the deposition of molten steel on the inside wall of the vacuum treatment vessel is hard to effectively prevent, because of the limited heating capacity of the heat source.
Japanese Patent Application Kokai (Laid-open) No. 64-217 discloses a process comprising injecting a combustible gas into molten steel in a vacuum treatment vessel while supplying an oxygen gas over the surface of molten steel bath in the vacuum treatment vessel at the same time, thereby heating the molten steel to a higher temperature, but it has such a problem that the C and H contents of the molten steel increase because of the injection of the combustible gas into the molten steel, and the structure and maintenance of an apparatus for injecting the combustible gas into the molten steel are complicated. According to the present inventor's knowledge, the flow rate of the combustible gas to be injected into the molten steel is limited, and thus it is hard to effectively prevent the deposition of molten steel on the inside wall of the vacuum treatment vessel.
Japanese Patent Application Kokai (Laid-open) No. 1-195239 discloses a plurality of gas combustion burners for sole use in the prevention of molten steel deposition on the inside wall of a vacuum treatment vessel, and also in remelting and removal of the deposited steel, and also discloses a lance provided with a plurality of burners, but handling of a plurality of gas combustion burners or a lance provided with a plurality of burners is troublesome, and it is hard to use the disclosed technics at not more than 100 Torr and it is also hard to heat the molten steel or refractories of the wall of the vacuum treatment vessel to a enough higher temperature.